Mycobacterium tuberculosis, the bacterium which causes tuberculosis (TB), remains a major cause of death in the world in spite of relatively effective treatments with multi-drug combinations due to the lack of treatment accessibility, HIV co-infection, and the lengthy treatment time. Combinations of multiple drugs are required to prevent the emergence of drug resistance and to effectively treat the infection. Drug sensitive TB is treated with isoniazid, rifampicin, pyrazinamide and ethambutol or streptomycin. Importantly, there are many side effects associated with the drugs making compliance difficult and tolerability challenging. For example, isoniazid causes peripheral nervous system disorder and induces serious liver dysfunction in some people when used in combination with rifampicin; rifampicin can cause liver dysfunction or hepatopathy, malaise, drug allergy, and its use with other drugs such as HIV protease inhibitors, is compromised due to P450-associated enzyme induction. In summary, tolerability remains a major challenge for current TB drug treatment regimens.
A revitalized effort to develop new drug treatments and combinations of drugs has been inspired by medical philanthropy and cooperative efforts of public private partnerships. In addition, in spite of the total number of cases of TB having decreased in most countries, the lack of accessibility of treatment of multi-drug resistant (MDR-TB) and extensively drug resistant (XDR-TB) combined with the emergence of extremely drug resistant (XXDR-TB) or totally drug resistant (TDR-TB) raises global concerns about the sustainability of the current control measures. Furthermore, there remain too few combinations that have the potential to shorten the treatment time of drug sensitive and drug resistant TB infection (currently from six months to two years, respectively), which remains critical to treatment compliance and preventing the emergence of drug resistance. Moreover, there is mechanistic and chemical redundancy for compounds currently in clinical development and safety liabilities for most classes of drugs reflecting a lack of bold and innovative approaches. For example, two fluroquinolones in phase 3 clinical trials do not address the problem of XDR-TB and may exacerbate drug resistance; three oxazolidinones in phase 2 clinical trials have not proven to shorten the time of treatment; and two nitroimidazoles in clinical development may be incompatible with some of the front line treatment drugs. Thus, there remains an overall dearth of drug candidates for the treatment of tuberculosis; see WGND pipeline as reference http://www.newtbdrugs.org/pipeline.php.
As a result of the need to combat drug-resistant bacteria and the increasing failure of the available drugs, there has been a resurgent interest in discovering new antibiotics. One attractive strategy for developing new antibiotics is to inhibit DNA gyrase and/or topoisomerase IV, bacterial enzymes necessary for DNA replication, and therefore, necessary for bacterial cell growth and division. Gyrase and/or topoisomerase IV activity are also associated with events in DNA transcription, repair and recombination. Gyrase is one of the topoisomerases, a group of enzymes which catalyze the interconversion of topological isomers of DNA (see generally, Kornberg and Baker, DNA Replication, 2d Ed., Chapter 12, 1992, W. H. Freeman and Co.; Drlica, Molecular Microbiology, 1992, 6, 425; Drlica and Zhao, Microbiology and Molecular Biology Reviews, 1997, 61, pp. 377-392). Gyrase itself controls DNA supercoiling and relieves topological stress that occurs when the DNA strands of a parental duplex are untwisted during the replication process. Gyrase also catalyzes the conversion of relaxed, closed circular duplex DNA to a negatively superhelical form which is more favorable for recombination. The mechanism of the supercoiling reaction involves the wrapping of gyrase around a region of the DNA, double strand breaking in that region, passing a second region of the DNA through the break, and rejoining the broken strands. Such a cleavage mechanism is characteristic of a type II topoisomerase. The supercoiling reaction is driven by the binding of ATP to gyrase. The ATP is then hydrolyzed during the reaction. This ATP binding and subsequent hydrolysis cause conformational changes in the DNA-bound gyrase that are necessary for its activity. It has also been found that the level of DNA supercoiling (or relaxation) is dependent on the ATP/ADP ratio. In the absence of ATP, gyrase is only capable of relaxing supercoiled DNA.
Bacterial DNA gyrase is a 400 kilodalton protein tetramer consisting of two A (GyrA) and two B subunits (GyrB). Binding and cleavage of the DNA is associated with GyrA, whereas ATP is bound and hydrolyzed by the GyrB protein. GyrB consists of an amino-terminal domain which has the ATPase activity, and a carboxy-terminal domain which interacts with GyrA and DNA. By contrast, eukaryotic type H topoisomerases are homodimers that can relax negative and positive supercoils, but cannot introduce negative supercoils. Ideally, an antibiotic based on the inhibition of bacterial DNA gyrase and/or topoisomerase IV would be selective for these enzymes and be relatively inactive against the eukaryotic type II topoisomerases.
Topoisomerase IV primarily resolves linked chromosome dimers at the conclusion of DNA replication.
The widely-used quinolone antibiotics inhibit bacterial DNA gyrase (GyrA) and/or Topoisomerase IV (ParC). Examples of the quinolones include the early compounds such as nalidixic acid and oxolinic acid, as well as the later, more potent fluoroquinolones such as norfloxacin, ciprofloxacin, and trovafloxacin. These compounds bind to GyrA and/or ParC and stabilize the cleaved complex, thus inhibiting overall gyrase function, leading to cell death. The fluoroquinolones inhibit the catalytic subunits of gyrase (GyrA) and/or Topoisomerase IV (Par C) (see Drlica and Zhao, Microbiology and Molecular Biology Reviews, 1997, 61, 377-392). However, drug resistance has also been recognized as a problem for this class of compounds (WHO Report, “Use of Quinolones in Food Animals and Potential Impact on Human Health”, 1998). With the quinolones, as with other classes of antibiotics, bacteria exposed to earlier compounds often quickly develop cross-resistance to more potent compounds in the same class. The associated subunits responsible for supplying the energy necessary for catalytic turnover/resetting of the enzymes via ATP hydrolysis are GyrB (gyrase) and ParE (topoisomerase IV), respectively (see, Champoux, J. J., Annu. Rev. Biochem., 2001, 70, pp. 369-413). Compounds that target these same ATP binding sites in the GyrB and ParE subunits would be useful for treating various bacterial infections (see, Charifson et al., J. Med. Chem., 2008, 51, pp. 5243-5263).
There are fewer known inhibitors that bind to GyrB. Examples include the coumarins, novobiocin and coumermycin A1, cyclothialidine, cinodine, and clerocidin. The coumarins have been shown to bind to GyrB very tightly. For example, novobiocin makes a network of hydrogen bonds with the protein and several hydrophobic contacts. While novobiocin and ATP do appear to bind within the ATP binding site, there is minimal overlap in the bound orientation of the two compounds. The overlapping portions are the sugar unit of novobiocin and the ATP adenine (Maxwell, Trends in Microbiology, 1997, 5, 102).
For coumarin-resistant bacteria, the most prevalent point mutation is at a surface arginine residue that binds to the carbonyl of the coumarin ring (Arg136 in E. coli GyrB). While enzymes with this mutation show lower supercoiling and ATPase activity, they are also less sensitive to inhibition by coumarin drugs (Maxwell, Mol. Microbiol., 1993, 9, 681).
Despite being potent inhibitors of gyrase supercoiling, the coumarins have not been widely used as antibiotics. They are generally not suitable due to their low permeability in bacteria, eukaryotic toxicity, and poor water solubility (Maxwell, Trends in Microbiology, 1997, 5, 102). It would be desirable to have a new, effective GyrB and ParE inhibitor that overcomes these drawbacks and, preferably does not rely on binding to Arg136 for activity. Such an inhibitor would be an attractive antibiotic candidate, without a history of resistance problems that plague other classes of antibiotics.
As bacterial resistance to antibiotics has become an important public health problem, there is a continuing need to develop newer and more potent antibiotics. More particularly, there is a need for antibiotics that represent a new class of compounds not previously used to treat bacterial infection. Compounds that target the ATP binding sites in both the GyrB (gyrase) and ParE (topoisomerase IV) subunits would be useful for treating various bacterial infections. Such compounds would be particularly useful in treating nosocomial infections in hospitals where the formation and transmission of resistant bacteria are becoming increasingly prevalent. Furthermore, there is a need for new antibiotics having a broad spectrum of activity with advantageous toxicological properties.